In lightweight construction, in particular in aircraft construction, use is increasingly being made of composite components made of fiber-reinforced plastics, which can withstand extreme mechanical loads and at the same time offer a high weight-saving potential. These components are formed with reinforcing fibers which are subsequently saturated or impregnated with a curable polymer material, for example a polyester resin, an epoxy resin or the like, to form the finished component.
The alignment of the reinforcing fibers in a component of this type has a decisive influence on its rigidity and strength. To achieve optimum mechanical properties, the reinforcing fibers should, if possible, follow the direction of loading and not have any wave formation. In addition, it is desirable for each individual reinforcing fiber to be subjected to uniform loading.
With conventional semifinished products, such as, for example, woven or laid fiber fabrics, for reinforcement of the polymer material, not all conceivable fiber orientations can be realized, since the reinforcing fibers are generally arranged there in a specific, fixed orientation. Although laid fiber fabrics can be “draped”, that is to say laid in a planar manner without creasing, for example to form segments of a circular ring, the reinforcing fibers generally cannot bring themselves into line with the path followed by more complex lines of force flux.
One possible way of complying with a requirement for fiber alignment in accordance with loading is the known TFP process. This involves the laying of fiber strands for mechanical reinforcement (“rovings”), which are in turn formed by a multiplicity of discrete reinforcing fibers running parallel to one another, along any desired path curve and attaching them with the aid of a fixing thread on a backing layer to form a fiber preform (“preform”), whereby the alignment of the individual fiber strands can be adapted virtually optimally to the forces acting on the finished composite component. The fixing is performed here by an upper fixing thread and a lower fixing thread, which are interlinked with one another underneath the backing layer—in a way corresponding to conventional sewing methods. The attachment of the fiber strands is preferably performed here with zigzag stitches. The optimum utilization of the mechanical load-bearing capacity of the fiber strands that is achieved in this way can minimize their number, and consequently also the weight. Moreover, the cross section of the component can be adapted in an ideal way to the respective local loads. Furthermore, reinforcements can be formed specifically in zones that are subjected to particular loading, such as, for example, regions where force is introduced or the like, by laying additional fiber strands. The discrete reinforcing fibers are formed, for example, by glass fibers, carbon fibers, aramid fibers or the like.
The production of fiber preforms by means of the TFP process is performed on customary CNC-controlled automatic sewing and embroidering machines, which are also used, for example, in the textile industry. Once all the required layers have been laid with fiber strands, the finished fiber preform, which generally already has the desired final contour, is placed in a closable mould, and impregnated with a curable polymer material and subsequently cured to form the finished composite component. A number of TFP fiber preforms and/or layers of reinforcing fabrics may be combined here. Multi-layered fiber preforms are formed by placing a number of (single, single-layered) fiber preforms one on top of the other, so as to be able to create greater material thicknesses that could not otherwise be produced on account of the limited needle length in the automatic sewing or embroidering machines that are used for the TFP process. Multi-layered fiber preforms accordingly have at least two backing layers, running approximately parallel to one another within the multi-layered fiber preform.
The impregnation of the fiber preforms with the curable polymer material may be performed, for example, by means of the known RTM process (“Resin Transfer Moulding”) in a correspondingly designed closable mould.
However, with the fixing thread and the backing layer, the TFP process introduces into the fiber preform two elements that no longer perform any function in the later composite component, in particular no backing function. Rather, both the backing layer and the fixing threads cause problems in realizing an ideal sequence of layers and, moreover, represent a not insignificant proportion of the overall weight, in particular if a number of fiber preforms are placed one on top of the other or single-layered fiber preforms of great material thickness are formed by a multiplicity of fiber strands lying one on top of the other. Although the backing layer itself may also be formed by a woven reinforcing fabric, for example by a woven glass- or carbon-fiber fabric, even in this case at least some of the reinforcing fibers have an alignment that is not in accordance with the loading. Moreover, the woven reinforcing fabric is also impaired by the penetration with the sewing needle during the TFT process, so that the characteristic material values may be impaired. In order to avoid the difficulties mentioned, the fixing threads may be formed, for example, by readily meltable material, but this results in an undefined amount of material entering the fiber preform, which may impair the mechanical properties of the matrix formed by means of impregnation with a curable polymer material in the later composite component.